Thermionic microphone



May 9, 1939.

F PULASKI 2,157,719

THERMIONIO'MICROPHONE Filed Sept 15, 1956 3nveutor Emil/6' L F 9uZws'ifi;

(Ittonleg May 9, 1939 UNITED STATES PATENT OFFICE THEBMIONIC MICROPHONEFrank L. Pulaski, Sherwood, Oreg.

Application September 15, 1936, Serial No. 100,810

2 Claims.

My invention relates to microphones. More particularly, my inventionrelates to a thermionic tube which is actuated by a diaphragm to therebyvary electronic currents within the tube in accordance with movements ofthe tube.

I am aware that microphones have been used which depend upon themovement of a voice coil within a magnetic field. The moving coil,cutting magnetic lines of force, establishescurrents which vary inproportion to the movements of the coil. In the present invention amagnetic field is utilized for establishing relative movements ofelectrodes within a themionic tube. In the preferred arrangement thethermionic tube moves with the diaphragm to which it is attached. Themagnetic field restrains the movement of a pair of shielding electrodes,thereby variably exposing the cathode of the tube to the influence ofthe anode.

My invention may be best understood by reference to the accompanyingdrawing in which Fig. 1 is a view, partly in section, of one embodimentof my invention,

Fig. 2 is a front elevational view of the apparatus illustrated in Fig.1,

Figs. 3 and 4 are sectional views, partly in elevation of one type ofthermionic tube which may be employed in my invention,

Fig. 5 is a plan view of the centering spider which is mounted withinthe tube, and

Fig. 6 is a schematic circuit diagram illustrating the connection of thethermionic tube.

Throughout the specification and drawing, similar reference numeralswill be used to indicate similar parts.

Referring to Fig. 1, a diaphragm supportinghousing I is attached to aU-shape permanent magnet 3. The housing may include suitable orifices 2which permit -unrestricted movements of the diaphragm. The larger end ofthe diaphragm supporting member terminates in a ring 5 on which ismounted a conical diaphragm l. corrugations 9 are included between thering 5 and the diaphragm 'I to provide flexibility. At the apex of theconical diaphragm is suitably secured a thermionic tube H which will behereinafter described in detail. A pair of spiders B3 are mounted inspaced relation with the vacuum' tube II. The spiders are connected attheir centers by a rod l5. The spiders l3 are constructed of a suitablespring material of a form which is. by way of example, illustrated inFig. 5. The circumferences of each of the spiders are secured to theinner surface of the envelope. A magnetic armature I1 is fixed to therod IS in a.

position opposite the pole pieces ill of the U- shape magnet. The tubeis mounted so that it will just clear the pole pieces l9 of thepermanent magnet 3.

Referring to Figs. 3 and 4, the thermionic tube 5 H is comprised of aglass, or non-ferrous metal, evacuated envelope 2i. Within the envelopeare mounted an anode 23 and a cathode 25 which is heated in theconventional manner. A pair of end plates 21 are mounted on oppositesides of the 10 cathode 25. The rod i5, which is fastened to the centersof the spiders i3, is extended to within the anode 23. The extendedportion of the rod includes a yoke 29. A pair of shielding electrodes 3|are mounted on the ends of the yoke. The 15 shielding electrodes and theend plates are preferably so constructed and arranged that the cathodeis substantially completely shielded from the anode when the tube isnormally disposed within the pole pieces.

A schematic circuit diagram, Fig. 6, illustrates one circuit arrangementwhich may be employed. The anode 23 is connected through an outputtransformer 33 and anode battery 35 to the oathode 25. The end plates 21and shielding electrodes 3| may be connected together and biasedslightly negative with respect to the cathode 25 by a battery 31 or thelike. The leads are made flexible to permit free movements of the tube.

Having described the elements of my invention, the theory of operationwill now be given. As the diaphragm is actuated, the magnetic fieldbetween the pole pieces i9 and the armature ll restrains movements ofthe armature but permits the tube, as a whole. to move. Such movementwill cause the envelope including the anode, end plates, and cathode tomove but the armature I1 and shielding electrodes will be restrainedfrom movement. Thus the cathode is exposed to the influence of theanode. m

Since the anode may be biased positively with respect to the cathode, itwill follow that electrons emitted from the cathode travel to the anode.The number of electrons will be regulated by the variable shieldingeilect of the shielding electrodes. Thus, as the diaphragm I isactuated, the thermionic tube II will move to and fro, carrying with itthe anode, cathode and end plates, while the shielding electrodes willbe maintained in the same position as their normal arrangement. In thismanner the shielding electrodes permit varying currents to fiow betweenthe cathode and the anode in proportion to the tube movements.

It should be understood that numerous modia suitable electromagnet.

flcations within the scope or my invention may be made by those skilledin the art. For example, the permanent magnet may be replaced by Itshould likewise be understood that the shielding electrodes may be madeof diiferent shapes and of difierent arrangements. A suitable spider,secured between the free end of the thermionic tube and the magnet 3,may be used to position the tube and to add -restoring force to thediaphragm. Although I have illustrated a conical diaphragm, it should beunderstood that numerous diaphragm arrangements may be used. Icontemplate that the novel arrangement of thermionic tube andrestraining field may be used in combination with actuating mechanismother than a diaphragm. It should be understood that these modicationsand others are within the scope and spirit of my invention which is onlylimited in accordance with the foregoing specification and appendedclaims.

I claim as my invention:

1. In a device of the character described, a diaphragm, a thermionictube including envelope, anode and cathode electrodes mounted within andin fixed relation to said envelope, shielding electrodes flexiblymounted within said envelope, means for causing said anode and cathodeelectrodes to move in response to movements of said diaphragm, and meansfor restraining the movement of said shielding electrodes, said meansineluding a magnet fixedly mounted external to said tube and having itslines of force passing through said tube in a region remote from saidelectrodes, an armature flexibly mounted within said envelope andlocated in the field of said magnet, and means operably connecting saidarmature and said shielding'electrodes so'that said magnet restrains themotion of said armature and ot said shielding electrodes.

2. In a device oi the character described, a diaphragm, a thermionictube including an envelope, anode and cathode electrodes mounted withinand in fixed relation to said envelope, shielding electrodes flexiblymounted within said envelope and positioned so as to partially blockelectrons flowing from said cathode to said anode; means for causingsaid anode and cathode electrodes to move in response to movements ofsaid diaphragm, means for restraining the movement of said shieldingelectrodes, said means including a magnet having opposing pole piecesfixedly mounted external to said tube and having its line oi! forcepassing through said envelope FRANK L. PULASKI.

